The present invention relates to an apparatus for suspending a starting member from a rotary shaft so as to support the starting member, which is used at the time of manufacture of glass preform suitable for production of an optical fiber.
For production of the optical fiber, VAD (vapor phase axial deposition) method is known. FIG. 4 shows VAD method schematically. In FIG. 4, minute glass particles 4 of SiO.sub.2 formed in oxy-hydrogen flame are deposited on a quartz rod 5 mounted on a distal end of a rotating quartz bar 3 so as to make a cylindrical porous preform and the porous preform is sintered so as to produce a transparent preform of the optical fiber. The quartz bar 3 and the quartz rod 5 act as the starting member.
FIG. 1 is a longitudinal sectional view showing a connecting portion between the quartz bar 3 and a rotary shaft 1 in a prior art apparatus, while FIG. 3 is a cross-sectional view of the connecting portion in the prior art apparatus. As shown in FIGS. 1 and 3, a cylindrical projection 3a is provided at an upper end of the quartz bar 3, while a cylindrical slot la for receiving the cylindrical projection 3a is formed at a lower end of the rotary shaft 1. A pin hole 1c further extends through a lower end portion of the rotary shaft 1 across the cylindrical slot 1a. After the cylindrical projection 3a of the quartz bar 3 has been fitted into the cylindrical slot 1a of the rotary shaft 1, the projection 3a is coupled with the rotary shaft i by driving a pin 2 into the pin hole 1c of the rotary shaft 1 through the projection 3a. In the prior art apparatus, a clearances between a diameter of the slot 1a and a diameter of the projection 3a is set at about 0.1 mm such that run-out of the glass preform at the time of rotation of the rotary shaft 1 is eliminated.
However, since the prior art apparatus for supporting the glass preform is subjected to thermal deformation due to its long-term use or a pin hole of the projection 3a and the rotary shaft 1 is deformed by weight of the preform, it is impossible to eliminate run-out of the preform.
Meanwhile, if run-out of the preform takes place, heat applied to the porous preform in a furnace becomes nonuniform. As a result, shrinkage speed of the porous preform becomes nonuniform when the porous preform is changed to the glass preform, so that the glass preform is deformed as shown in, for example, FIG. 5. In FIG. 5, the glass preform not only has an effective portion A but a bent ineffective portion B, which is undesirable. This phenomenon is particularly conspicuous in the case where the porous preform is deposited around the quartz rod by outside vapor phase deposition method and is sintered.